Yet another thread - 12v reads 11.8v when sitting, and 11.2v under load

Discussion in 'Gen II Prius Care, Maintenance and Troubleshooting' started by arewethereyet, Mar 8, 2012.

  1. arewethereyet

    arewethereyet Junior Member

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    Information:

    • 2005 Prius with 99k
    • Not sure if battery has been replaced since it's new to me
    • My normal daily drive is 40 minutes total (22 miles)
    • 43mpg with winter tires previously inflated to 27 PSI (now running 38 PSI)
    • I'm not encountering any issues at this time (unless you think 43MPG is bad for winter), just checking battery health
    I checked the voltage last night after being idle for about 4 hours, 11.8v, then 11.5v with ACC on
    I checked the voltage this morning after idle all night and it read 11.8v, then dropped to 11.2v with ACC on.

    I realize it should read around 12.3-12.5.

    Is it possible battery is okay and I just need to supplement the charging of the 12v?

    My gut says order an optima 12v, just looking for some feedback from people in the know...
     
  2. jdenenberg

    jdenenberg EE Professor

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    You drive enough to keep the 12v battery charged and yours is not. Get a new one installed (and yes the Optima is better than the one Toyota installs).

    On a side note, 27 PSI is too low for good MPG and you are due to change out the coolant in both loops (engine and inverter). You should also have put in new ATF in your Prius transmission by now (even though Toyota says that it is not necessary).

    JeffD
     
  3. arewethereyet

    arewethereyet Junior Member

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    Thanks for the info. Since I just got the car, I'm making a list of what I want to do to it.

    Transmission is already on my list based on my reading. I'll read up on coolant and will most likely add that as well.
     
  4. edthefox5

    edthefox5 Senior Member

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    12 volt is toast and is also hurting your mileage. Engine runs alot more to charge it.

    BTW welcome to PC. And if you want to thank peeps for there replies just click the Thanks button on the lower right hand side of there post.
     
  5. The Critic

    The Critic Resident Critic

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    Have you had the battery tested by a Midtronics tester? This is the only tester that battery manufacturers approve of for warranty claims.

    Personally I still am not convinced that these voltage numbers are truly indicative of battery condition. It seems that the Prius has a difficult time maintaining a full charge on the 12v battery under certain driving conditions, but this does not necessarily mean the battery is bad.
     
  6. Cory151

    Cory151 Junior Member

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    You can get a load tester from Harbour Freight for less than $20 which will put a load on the battery via a heated element that will more accurately access the the battery status. If you throw it on a load tester and it hits 12v and begins to slide down the scale slowly its gone. Replace it.
     
  7. Britprius

    Britprius Senior Member

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    This test is fine provided you have charged the battery first, otherwise it will give a false reading of a dead battery.
     
  8. arewethereyet

    arewethereyet Junior Member

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    I replaced the battery today with an optima. Optima was 12.4 out of the box, but the display in the car showed 12.0. I'm going to see what it looks like after a week or so of driving. Hopefully it's up around the 12.3-12.4 mark.
     
  9. Patrick Wong

    Patrick Wong DIY Enthusiast

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    A new, fully-charged AGM 12V battery should measure ~12.9V. What are you using to measure voltages?

    If you are confident that your digital meter is accurate then I suggest that you give your new battery a break by fully charging it now. At 12.4V, the battery is substantially discharged.
     
  10. uart

    uart Senior Member

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    Was that 12.0 in Acc mode with all other loads off? (radio fans interior/exterior lights all off)
     
  11. edthefox5

    edthefox5 Senior Member

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    Exactly what Pat said.

    Btw, the best battery tester in the world for a shade tree mechanic is a $69 battery charger in 2 amp mode.

    Here's the 2 things it will tell you:

    1---How much current it pulls when you begin the charge.
    That tells me how discharged the battery is.

    2---How healthy the battery is by how long it takes to fully charge the battery. (Till it reaches 0 current draw). Or if it reaches 0 current draw at all. A mutt will never fully charge.

    Once a month maint. charge and you will become very familiar how healthy the battery is over the course of its life. If it starts to take forever to fully charge or won't fully charge..... look out.
    Takes 10 mins.
     
  12. PapaGallo

    PapaGallo Junior Member

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    just tested my battery today, 12.5 with engine off, 14.7 with engine on.
    Original battery, 29 months- had to jump start twice in those 29 months.
    Do i need to replace this battery? tested the battery with a Midtronics battery tester.
     
  13. The Critic

    The Critic Resident Critic

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    If the battery tested fine with a midtronics tester, there is absolutely no reason to replace it.
     
  14. PapaGallo

    PapaGallo Junior Member

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    Thanks , I was a bit concerned when I read Patricks response to OP concerning the voltage of the replacement battery being at 12.4V "At 12.4V, the battery is substantially discharged".
    The midtronics was a pbt-200 model, the result was "OK"
     
  15. The Critic

    The Critic Resident Critic

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    The midtronics tester is the only form of testing that is approved by battery manufacturers for warranty purposes.

    The voltage test mentioned by others is....well not industry approved.
     
  16. uart

    uart Senior Member

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    A question for the people reporting 12.9 volts for their fully charged batteries. How long after removing the battery from the charger are you measuring this? Most of the regular (flooded) lead acid batteries that I've dealt with have a fully charged rest voltage of about 12.6 to 12.7 volts.

    People also need to remember that when they test the battery voltage in the car via the service menu that there's a couple of amps drain to power the prius systems (even in acc mode), so you can expect 0.2 to 0.3 volts less than the ideal rest voltage.
     
  17. Patrick Wong

    Patrick Wong DIY Enthusiast

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    It is true that a flooded 12V battery has a fully-charged voltage of ~12.6V. However I am not talking about a flooded battery. I am talking about an AGM battery which produces a slightly higher voltage.

    Further, the 12.9V measurement that I am referring to is based upon a new AGM battery out of the box, which had not been charged since it was put into the box by the battery factory in Japan, a few months prior to my receiving it. I had posted about this several months ago along with a photo of my Fluke 87 DMM showing the voltage reading.
     
  18. uart

    uart Senior Member

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    Yeah I knew that you were referring to AGM, that's why I made the point of specifying that I only had data on the "regular (flooded)" type.

    So your 12.9 was the o/c measurement on the OEM as shipped. I wonder what these optima yellow tops should be shipping at then? Optima refers to them as "Spiralcell AGM", so you'd think they'd be similar in terms of fully charged resting voltage to that of the OEM..

    So has anyone here ever received one of the yellow tops with around 12.8 to 12.9 volts? Is it that we only hear about the lowish voltage ones (and never the good ones) here?
     
  19. dhanson865

    dhanson865 Expert and Devil's advocate

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    As I mentioned in http://priuschat.com/forums/gen-ii-...y-doa-almost-getting-worse-2.html#post1493405

    My yellowtop arrived at 12.52 volts, and after charging for 18 hours was 12.94 volts.

    I only let it sit off the charger for a couple of hours but I didn't see the voltage drop from the 12.94 in that time. I wanted use of the car so I didn't leave it off the charger long enough to see what the self discharge rate was.